How can I minimize diacetyl in my lagers?

Dear Mr. Wizard,

I have just started brewing lager beers, but have had problems with my first batches. The problem is the production of diacetyl. I just can’t seem to get rid of it. I believe that I am following good lagering technique, but my beer tastes like a butterscotch sundae. Here is what I am doing: After wort production, 5 gallons (19 L) total, I am chilling the wort to 52 ºF (11 ºC) overnight. I then pitch a 2-quart (~2-L) slurry of yeast, the strain is Wyeast 2007 Pilsen Lager Yeast. I ferment for two weeks at 52 ºF (11 ºC), then raise the temperature to 60 ºF (16 ºC) for three to four days to finish the fermentation. I chill the beer down to 32 ºF (0 ºC) at a rate of 4 ºF (~2 ºC) per day. I rack the beer to a keg for final lagering and lager at 32 ºF (0 ºC) for four weeks. The problem is that I cannot taste the diacetyl in the green beer, I can only detect it after the lagering period. I am trying to salvage this batch by depressurizing the keg, pitching a fresh slurry with a little extra corn sugar to feed the yeast as a makeshift kraüsening method, then refermenting at 60 ºF (16 ºC) for a couple of weeks and relagering. I am fairly experienced with sanitation, but I cannot completely rule out contamination without a microscope. I have produced many good ales with no contamination in the past. Is there any hope or am I an ale drinker for the rest of my days?

Darrin Burchell
Paris, Kentucky

Mr. Wizard replies:

The first thing that comes to mind when contemplating diacetyl problems is yeast strain. I have not personally used Wyeast 2007 Pilsen Lager Yeast, but after reading its description and learning that it is not highly flocculent, I don’t believe this yeast is prone to diacetyl-laden beers. Highly flocculent yeast strains are often associated with diacetyl problems because the yeast drops out of solution before the diacetyl in the beer is reduced.

The observation you make about detecting no diacetyl in the green beer before racking but picking it up after is important. You indicate that you ferment the beer to completion, raise the temperature for a diacetyl rest, chill to 32 ºF (0 ºC) then rack it to lagering. I think what is happening is that you are picking up some oxygen during your racking step and the oxygen converts diacetyl precursor (i.e. alpha acetolactate) in the green beer into diacetyl during lagering. You have already cooled the beer to 32 ºF (0 ºC) and the yeast is inactive.

Another possible cause is bacterial growth that occurs slowly and shows up later in the process. If you maintain effective sanitation practices and ferment with healthy yeast, this is probably not the problem. Yet another possibility is that you only detect the diacetyl late in the process when some of the “funky” flavors associated with fermenting and young beer have mellowed; the diacetyl was present before you racked to lagering, but you were unable to taste it.

Let’s get back to the “late blooming” diacetyl. This is actually quite common and is caused by prematurely chilling the beer. What happens is that diacetyl precursor remains in the beer after the yeast has been effectively knocked out by chilling. Any oxygen or oxidizing ions like iron or copper can later convert the precursor to diacetyl and there will not be any yeast left to mop it up. Kraüsening is definitely
an effective technique to correct diacetyl problems associated with rushed fermentation . . . bacterial problems however cannot be mended by this technique.

Most brewers who kraüsen add about one part kraüsen beer to nine parts finished beer. The key to the method is having an actively fermenting population of yeast. The high kraüsen stage of fermentation is the peak of excitement — for ales this occurs about 24 hours after pitching and for lagers about 48–72 hours after pitching (both durations are heavily influenced by yeast health and pitching rate). You are on the right track with your proposed remedy, but adding corn sugar and yeast to finished beer will not rapidly kick-start the uptake of diacetyl.

If you are going to take the time to save your buttery brew, spend a little time making a kraüsen beer. I would suggest making 2 quarts (~2 L) of wort using dry malt extract and a sprinkle of hops to get the bitterness in the same ballpark as your troubled batch. If I were doing this I would boil the wort for 30-60 minutes, transfer to a gallon jug, screw on the lid and throw it in the fridge or in an ice bath to cool it down. Such a small volume will be easy to cool without a wort chiller.

Once cooled to about 68 ºF (20ºC), add
a pack of Wyeast 2007 Pilsen Lager Yeast and start the propagation. At this time, move your keg out of cold storage and allow it to warm up to around 60 ºF (15 ºC) in preparation for the yeast. Keep an eye on the progress of the starter and when it kicks into high gear (i.e. approaches high kraüsen), transfer this to your keg. Hold at 60 ºF (15 ºC) for 1–2 weeks and move it back into cold storage. You may want to rack before transferring back to the cold to remove the yeast added with the kraüsen beer.

I do not think lager fermentations are difficult once you get the hang of them. My personal experience with lagers has taught me to stay on guard and never make assumptions. The key things that I stay focused on with lager brewing is pitching rate, wort aeration, fermentation temperature and tracking the fermentation with a hydrometer. I used to base my decisions largely on time and by nose — but was burned several times by poor decisions. Your process description is solid except you omit the key confirmatory piece of data — specific gravity. Fermentation should be complete in two weeks, but without taking a gravity sample you will blindly go to the next process step only to be disappointed to find out it’s too late.

Ales are often a different story. Frequently there is absolutely no doubt when fermentation begins because yeast is flowing from the blow-off like lava oozing from an active volcano. Most ale strains pack up and head south after fermentation and the yeast floating on top of the fermenter vanishes and sinks to the bottom of the fermenter. Hydrometer checks with these types of fermentations are not nearly as critical but the diligent brewer will check just to confirm.

There is an old saying in carpentry, measure twice, cut once. The same idea can be applied to stepping through the stages of lager fermentation. Don’t be discouraged in
the future!

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